DC motors of the "inside-out" design typically comprise a permanent magnet rotor and a number of stator-mounted, electrically energizeable coils, the ends of which are connected to an array of commutator bars. Commutation may be effected by means of rollers as shown by my previously-identified applications and patents or conventional brush commutation techniques may be employed, for example a face plate or disc-type system as disclosed in Goraszko U.S. Pat. No. 3,275,861.
Traditionally, the permanent magnet members of DC motors of both conventional and inside-out designs have been alnico or ceramic magnets. Following the advent of rare earth magnets, with their extremely high energy product and intrinsic coercive force, Rollin James Parker suggested in U.S. Pat. No. 3,836,802 that they be combined with alnico magnets in the stator of a conventional DC motor. In several of my previously-identified applications I have shown how rare earth magnets could be combined with conventional magnets to enhance the flux density in the air gap of a DC motor of the inside-out design.
Rare earth magnets, as such, are very difficult to work with because of the extremely strong fields they create. Thus, one of their greatest virtues is at the same time one of their greatest liabilities. It is simply not practical to machine rare earth magnets or structures containing such magnets because the machined particles adhere so strongly to the magnets. Because of the extremely high coercive forces of rare earth magnets it is also not practical to construct a rotor or stator of rare earth magnets and then remagnetize the structure as is commonly done with alnico magnets. Once again, one of the greatest virtues of rare earth magnets, i.e., the extremely high resistance to demagnetization, is also one of the greatest liabilities.
Rare earth magnets are not generally available in the sizes of conventional magnets, i.e., they are typically furnished in the size of about 1/2 inch .times. 1/2 inch .times. 1 inch or smaller. When used with conventional magnets as part of a permanent magnet rotor in a DC motor of the inside-out design as shown in my earlier-identified applications they are glued to the rotor structure, a less than satisfactory mechanical bonding arrangement for high speed operation.
Applicant has overcome these and other difficulties associated with the use of rare earth magnets and has provided a rotor containing rare earth magnet material, which rotor can be magnetized after assembly and machining. Applicant has also developed a method for manufacturing such a rotor comprising the steps of: (1) assembling a stack of steel rotor laminations having slots adapted to receive pieces of rare earth magnet material; (2) inserting pieces of virgin rare earth magnet material in the slots; (3) positioning a shaft in the center of the stack; (4) casting a rotor comprising the laminations, the rare earth magnet material and the shaft with aluminum; (5) machining the rough-cast rotor; and (6) then magnetizing the rare earth magnet material in situ. The strength of the rare earth magnets may be adjusted or "trimmed" by heating the rotor in the absence of iron which would complete the flux path of the rare earth magnets, thereby reducing the strength of the field. The strength of the rare earth magnets can then be increased by magnetizing in the same direction as that used to originally magnetize the virgin rare earth magnet material in the cast rotor.
The rotor of the present invention, because of its ruggedness, structural integrity, high magnetic field and resistance to demagnetization, can be used for generators, synchronous machines and other electromagentic machines required to convert electrical energy to mechanical energy or vice versa.